What you're hearing is often called a handshake, the start of a telephone conversation between two modems. The modems are trying to find a common language and determine the weaknesses of the telephone channel originally meant for human speech.

In layman, it's similar to how a morse code message would sound - since you're always trying to connect to the same service on the same modem, the sounds are the same (there may be small differences during the handshake protocol {where both machines talk to each other and decide HOW to talk to each other}, but the general sequence of tones is the same).